Signal transducers and activators of transcription (STATs) are transcription factors that were discovered as key signaling components involved in mediating responses to cytokine stimulation. Increasingly, evidence has been accumulating that points to the movement of STAT proteins in oncogenesis. Previously, we made the original observation that one STAT family member, STAT3, is constitutively activated in fibroblast cell lines transformed by the Src oncoprotein. Studies with dominant-negative STAT3 proteins further demonstrate that activation of STAT3 signaling by the Src oncoprotein leads to specific regulation of gene expression and is required for cell transformation. In addition, STAT3 signaling is activated with high frequency in cell lines and by others suggest that STAT3 signaling has an important role in breast carcinoma and various lymphoid malignancies. In this project, we propose to test the hypothesis that specific inhibitors of STAT3 signaling will reduce tumorigenicity in relevant human tumor models without inducing significant toxicity. This goal will be pursued through the following specific aims: (1) To identify candidate lead peptides that potentially disrupt STAT3 function, we will screen linear and cyclic peptide phage display libraries for sequences that bind to STAT3 protein. (2) Candidate lead peptides, together with peptidomimetics and combinatorial chemical libraries synthesized in Project 1, will be tested in vitro assays for their abilities to disrupt STAT3 dimerization and DNA- binding activity. Information on activities peptides and other compounds will be fed back to Project 1 for lead optimization. (3) Promising compounds will be assayed for their abilities to disrupt STAT3 signaling in intact cells and to block cell transformation by the Src oncoprotein. (4) Using cell culture and animal models of human cancers where STAT3 is frequently activated, compounds that disrupt STAT3 signaling will be evaluated for their anti-tumor efficacy and toxicity. In addition, these STAT3 disruptors will be used to explore the biological mechanisms of how STAT3 contributes to oncogenesis, including regulation of cell proliferation and apoptosis. Results of these studies will advance our understanding of the role of STAT3 in human cancer and assess the therapeutic potential of STAT3 inhibitors as novel anti-cancer drugs.